Automated payment machines are commonplace, in applications ranging from soft drinks vending to high stakes gambling. Such machines must be capable of reliably authenticating notes and coins in a range of conditions with no human interaction.
To deter would-be thieves of bank notes in transit, it has become commonplace for notes to be stored and transported in secure containers which release an indelible dye or ink onto the bank notes in the event that the security container is stolen or otherwise interfered with, to mark and stain the bank notes, thus rendering them worthless and unusable. It is usually not possible to clean bank notes which have been marked in this way without damaging their original printing.
In addition to banknotes being purposely stained, it is possible for notes to become marked or degraded accidentally through spillage, general wear and graffiti.
Certain currency issuing authorities have mandated that security stained notes deemed unfit for circulation are neither accepted in a transaction nor returned to the customer, but are instead retained so as to be removed from circulation. This is easy to achieve in transactions involving a human cashier, where the cashier can physically inspect the note, but is more difficult to achieve with the certainty necessary when bank notes are used in automated payment machines.
Although automated payment machines typically include note validators which use a comprehensive suite of tests to determine the authenticity of a bank note, few have the ability reliably to detect notes that have been degraded specifically by staining ink. As a result a growing problem is the use of automated payment machines to convert large quantities of stained notes into clean notes that can then be used with impunity.
Current note validator systems may include visible, ultraviolet and infrared imaging devices as well as magnetic and capacitance sensing technologies to validate banknotes and also detect stains. However, existing techniques are often unable to determine the nature of the stain or accurately compute the affected area. To improve reliability additional additives such as those with infrared absorption characteristics can be added to the staining ink to improve machine detection. These additives however significantly increase the price of the inks.
Accordingly, it would be desirable to provide a system that can reliably determine the extent of staining on a bank note and whether such staining is likely due to security staining inks.